The widespred use of electric countershock in the control of cardiac arrhythmias has disclosed that this procedure is often accompanied by signs of myocardial injury such as; arrhythmias not present before, ECG changes, and decrease of contractile force. The principal goal of this study is an understanding of the underlying mechanisms of the injury. This information will be utilized to develop and test experimental modifications directed towards improving clinical defibrillation and cardioversion procedures. Methodology of this study will consist of determinations of current- induced changes of cellular characteristics such as membrane impedance, transmembrane potentials, intra-cellular H ion, and K ion and Na ion ion concentrations, amplitude of contraction, and electron micro-scopic ultra-structure. Knowledge of those parameters that change simultaneously will be interpreted in terms of cellular models to derive a unified picture of the mechanism of injury. Specific hypotheses concerning associated physico-chemical influences such as high temperature, direct structural damage due to electrical gradients across the membrane, and large shifts of ion distribution will also be tested. To facilitate measurement and control of current densities and cellular parameters, the experimental studies will be carried out in a tissue culture of myocardial cells. The influence of environmental factors that may be associated with the clinical procedures such as; hypoxia, electrolyte imbalance, and the presence of antiarrhythmic drugs as well as techniques for reducing cellular injury that may stem from early portions of the study will be investigated with the same preparation.